As discussed several times before, for recalls they use all the available info - from DVLA and the importers own records of owners/purchasers. The idea being that it's better to hit a few ex-owners with unnecessary letters than risk missing current ones who might have been a bit lax in notifying DVLA.
Fair enough, but it was a vehicle I sold 4 months ago and for which I've received a confirmation letter that I no longer own it from DVLA



